Method for fabricating a layered structure using wafer bonding

ABSTRACT

Methods and techniques for fabricating layered structures, such as capacitive micromachined ultrasound transducers, as well as the structures themselves. The layered structure has a membrane that includes a polymer-based layer and a top electrode on the polymer-based layer. The membrane is suspended over a closed cavity and may be actuated by applying a voltage between the top electrode and a bottom electrode that may be positioned along or be a bottom of the closed cavity. The layered structure may be fabricated using a wafer bonding process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of international application no.PCT/CA2018/051618, filed on Dec. 18, 2018, and entitled “LayeredStructure and Method for Fabricating Same”, which claims priority toU.S. provisional patent application No. 62/607,641, filed on Dec. 19,2017, and entitled “Layered Structure and Method for Fabricating Same”,the entireties of both of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure is directed at methods, systems, and techniquesfor fabricating a layered structure, such as a capacitive micromachinedultrasound transducer, and the structure itself.

BACKGROUND

Ultrasound imaging is the most widely used medical imaging modality inthe world in terms of images created annually. Ultrasound is useful forgenerating images of a variety of different targets within the humanbody. It is important that images are acquired with high quality and inan accessible, cost-effective manner since ultrasonic imaging has manymedical uses. The ultrasound transducer is the key hardware involvedsending and receiving ultrasonic waves to and from the body.Consequently, there exists a continued need to improve the capabilitiesof the transducer.

SUMMARY

According to a first aspect, there is provided a method for fabricatinga capacitive micromachined ultrasound transducer, the method comprising:depositing a sacrificial layer on a substrate assembly that functions asa bottom electrode; patterning the sacrificial layer to be shaped as acavity of the transducer; depositing a first polymer-based layer on thesacrificial layer; patterning a via hole through the first polymer-basedlayer to the sacrificial layer; patterning a top electrode on the firstpolymer-based layer above the sacrificial layer; depositing a secondpolymer-based layer on the top electrode such that the top electrode isbetween the first and second polymer-based layers; using the via hole,etching away the sacrificial layer to form the cavity of the transducer;and closing the cavity.

The top electrode may be embedded within the first and secondpolymer-based layers.

Patterning the sacrificial layer to be shaped as the cavity of thetransducer, etching away the sacrificial layer to form the cavity of thetransducer, and patterning the via hole, may be performed using organicand non-toxic solvents.

The first polymer-based layer may be photosensitive, and patterning thevia hole may comprise: cross-linking a portion of the firstpolymer-based layer to remain following the patterning by exposing theportion to ultraviolet radiation; and applying a photoresist developerto etch away the first polymer-based layer that is blocking the viahole. The portion to remain may exclude the first polymer-based layerblocking the via hole.

The sacrificial layer may not be photosensitive and patterning thesacrificial layer to form the cavity may comprise: cross-linking aportion of the sacrificial layer shaped as the first shape; and applyinga developer to etch away a portion of the sacrificial layer that is notcross-linked.

The sacrificial layer may not be photosensitive and patterning thesacrificial layer to be shaped as a cavity of the transducer maycomprise: depositing a positive photoresist layer on the sacrificiallayer; cross-linking a portion of the positive photoresist layercorresponding to a portion of the sacrificial layer that is shaped asthe cavity; applying a photoresist developer to etch away thephotoresist layer that is not cross-linked and the sacrificial layerthat underlies the photoresist layer that is not cross-linked; and afterthe photoresist layer and the sacrificial layer have been etched away,removing the photoresist layer that is cross-linked.

The second polymer-based layer may be thicker than the firstpolymer-based layer.

The second polymer-based layer may be at least five times thicker thanthe first polymer-based layer.

The relative thickness of the second polymer-based layer to the firstpolymer-based layer may be selected such that the top electroderesonates at a frequency of at least 1 MHz.

The fabrication may be performed at a temperature of no more than 150°C.

The substrate assembly may be flexible.

The top electrode may comprise a conductive polymer.

The substrate assembly may comprise an optically-transparent material.

The substrate assembly may further comprise an optically-transparentconductive bottom electrode on the substrate.

Closing the cavity may comprise encapsulating the first and secondpolymer-based layers with a bio-compatible material.

The bio-compatible material may comprise a poly(p-xylylene) polymer.

Patterning the top electrode may comprise patterning metallicconnections to the top electrode. The metallic connections may beuncovered by the second polymer-based layer after the cavity is closed,and closing the cavity may comprise depositing the bio-compatiblematerial over the metallic connections.

Closing the cavity may be done in a polymer evaporator chamber atpressure of no more than 0.001 Torr.

After the sacrificial layer has been etched away, trapping charge in thefirst polymer-based layer may be done by: applying a voltage across thetop electrode and the substrate assembly such that a portion of thefirst polymer-based layer contacting the top electrode may be pulledinto contact with the substrate assembly; maintaining the portion of thefirst polymer-based layer contacting the top electrode and the substrateassembly in contact for a period of time; and then ceasing applying thevoltage.

The sacrificial layer may comprise a polymer.

Depositing the sacrificial layer may comprise spray coating the polymercomprising the sacrificial layer on to the substrate assembly.

Depositing the first polymer-based layer on the sacrificial layer maycomprise covering all surfaces of the sacrificial layer except a bottomsurface contacting the substrate assembly with the first polymer-basedlayer.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

Closing the cavity may comprise forming a watertight seal around thecavity.

The sacrificial layer may be non-reactive when exposed to the first andsecond polymer-based layers and to a photoresist developer used duringthe patterning of the first and second polymer-based layers, and thefirst and second polymer-based layers may be non-reactive when exposedto an etchant used to etch away the sacrificial layer.

The first and second polymer-based layers may comprise SU8 photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

The cavity may have a height selected such that an operating voltage ofthe transducer is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

Depositing the sacrificial layer may comprise evaporating a compositionthat comprises a solvent, and then depositing the composition as thesacrificial layer. At least 70% and no more than 90% of the solvent maybe evaporated.

According to another aspect, there is provided a method for fabricatinga capacitive micromachined ultrasound transducer, the method comprising:depositing a first polymer-based layer on a substrate assembly thatfunctions as a bottom electrode; patterning the first polymer-basedlayer to be a cavity of the transducer; depositing a sacrificial layeron a separate substrate; depositing a second polymer-based layer overthe sacrificial layer; depositing a top electrode on the secondpolymer-based layer; depositing a third polymer-based layer on the topelectrode such that the top electrode is between the second and thirdpolymer-based layers; adhering the first and third polymer-based layerstogether such that the cavity is closed; and etching away thesacrificial layer such that the second polymer-based layer is releasedfrom the separate substrate.

The top electrode may be embedded within the second and thirdpolymer-based layers.

The method may further comprise cross-linking the first and thirdpolymer-based layers prior to adhering the first and third polymerlayers together.

Patterning the first polymer-based layer and etching away thesacrificial layer may be performed using organic and non-toxic solvents.

The first polymer-based layer may be photosensitive, and patterning thefirst polymer-based layer to be the cavity of the transducer maycomprise: cross-linking a portion of the first polymer-based layer toremain following the etching by exposing the portion to ultravioletradiation; and applying a photoresist developer to etch uncrossed-linkedareas of the first polymer-based layer.

The second polymer-based layer may be thicker than the thirdpolymer-based layer.

The second polymer-based layer may be at least five times thicker thanthe third polymer-based layer.

Relative thickness of the second polymer-based layer to the thirdpolymer-based layer may be selected such that the top electroderesonates at a frequency of at least 1 MHz.

The fabrication may be performed at a temperature of no more than 150°C.

The substrate assembly may be flexible and bonded to a rigid carrier.

The top electrode may comprise a conductive polymer.

The substrate assembly may comprise an optically-transparent material.

The substrate assembly may further comprise an optically-transparentconductive bottom electrode on the substrate.

Adhering the first and third polymer layers together may comprise:treating surfaces of the first and third polymer layers to be adhered toeach other with plasma; aligning the surfaces to each other; andpressing the surfaces together.

The adhering may be done in a bonding chamber at pressure of no morethan 0.001 Torr.

The method may further comprise, after the adhering, trapping charge inthe first polymer-based layer by: applying a voltage across the topelectrode and the substrate assembly such that a portion of the firstpolymer-based layer contacting the top electrode is pulled into contactwith the substrate assembly; maintaining the portion of the firstpolymer-based layer contacting the top electrode and the substrateassembly in contact for a period of time; and then ceasing applying thevoltage.

The sacrificial layer may comprise a polymer.

Depositing the second polymer-based layer on the sacrificial layer maycomprise completely covering the sacrificial layer with the secondpolymer-based layer.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

Following the adhering a watertight seal may be around the cavity.

The sacrificial layer may be non-reactive when exposed to the secondpolymer-based layer and to a photoresist developer used during thepatterning of the second polymer-based layer. The second polymer-basedlayer may be non-reactive when exposed to an etchant used to etch awaythe sacrificial layer.

The first, second, and third polymer-based layers may comprise SU8photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

The cavity may have a height selected such that an operating voltage ofthe transducer is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

Depositing the sacrificial layer may comprise evaporating a compositionthat comprises a solvent, and then depositing the composition as thesacrificial layer. At least 70% and no more than 90% of the solvent maybe evaporated.

According to another aspect, there is provided a method for fabricatinga layered structure, the method comprising: depositing a sacrificiallayer on a substrate assembly that functions as a bottom electrode;patterning the sacrificial layer into a first shape; depositing a firstpolymer-based layer on the sacrificial layer; patterning a top electrodeon the first polymer-based layer above the sacrificial layer; depositinga second polymer-based layer on the top electrode such that theelectrode is between the first and second polymer-based layers; andetching away the sacrificial layer to form a cavity under the electrode.

The electrode may be embedded within the first and second polymer-basedlayers.

The first shape may comprise a conduit extending through firstpolymer-based layer and permitting etchant to flow from a top of thefirst polymer-based layer to the sacrificial layer.

The second polymer-based layer may be deposited to block the conduit,and the method may further comprise etching away a portion of the secondpolymer-based layer that blocks the conduit. Etching away thesacrificial layer may comprise flowing the etchant through the conduit.

Patterning the sacrificial layer into the first shape and etching awaythe sacrificial layer to form the cavity may be performed using organicand non-toxic solvents.

The sacrificial layer may not be photosensitive and patterning thesacrificial layer to form the cavity may comprise: cross-linking aportion of the sacrificial layer shaped as the first shape; and applyinga developer to etch away a portion of the sacrificial layer that is notcross-linked.

The sacrificial layer may not be photosensitive and patterning thesacrificial layer to form the cavity may comprise: depositing a positivephotoresist layer on the sacrificial layer; cross-linking a portion ofthe positive photoresist layer corresponding to a portion of thesacrificial layer that is shaped as the cavity; applying a photoresistdeveloper to etch away the photoresist layer that is not cross-linkedand the sacrificial layer that underlies the photoresist layer that isnot cross-linked; after the photoresist layer and the sacrificial layerhave been etched away, removing the photoresist layer that iscross-linked.

The second polymer-based layer may be thicker than the firstpolymer-based layer.

The second polymer-based layer may be at least five times thicker thanthe first polymer-based layer.

Relative thickness of the second polymer-based layer to the firstpolymer-based layer may be selected such that the top electroderesonates at a frequency of at least 1 MHz.

The fabrication may be performed at a temperature of no more than 150°C.

The substrate assembly may be flexible.

The top electrode may comprise a conductive polymer.

The substrate assembly may comprise an optically-transparent material.

The substrate assembly may further comprise an optically-transparentconductive bottom electrode on the substrate.

The method may further comprise closing the cavity.

Closing the cavity may comprise encapsulating the first and secondpolymer-based layers with a bio-compatible material.

The bio-compatible material may comprise a poly(p-xylylene) polymer.

Patterning the top electrode may comprise patterning metallicconnections to the top electrode. The metallic connections may beuncovered by the second polymer-based layer after the cavity is closed,and closing the cavity may comprise depositing the bio-compatiblematerial over the metallic connections.

Closing the cavity may be done in a polymer evaporator chamber at apressure of no more than 0.001 Torr.

Closing the cavity may comprise forming a watertight seal around thecavity.

The method may further comprise, after the sacrificial layer has beenetched away, trapping charge in the first polymer-based layer may occurby: applying a voltage across the top electrode and the substrateassembly such that a portion of the first polymer-based layer contactingthe top electrode is pulled into contact with the substrate assembly;maintaining the portion of the first polymer-based layer contacting thetop electrode and the substrate assembly in contact for a period oftime; and then ceasing applying the voltage.

The sacrificial layer may comprise a polymer.

Depositing the sacrificial layer may comprise spray coating the polymercomprising the sacrificial layer on to the substrate assembly.

Depositing the first polymer-based layer on the sacrificial layer maycomprise covering all surfaces of the sacrificial layer except a bottomsurface contacting the substrate assembly with the first polymer-basedlayer.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

The sacrificial layer may be non-reactive when exposed to the first andsecond polymer-based layers and to a photoresist developer used duringthe patterning of the first and second polymer-based layers, and thefirst and second polymer-based layers may be non-reactive when exposedto an etchant used to etch away the sacrificial layer.

The first and second polymer-based layers may comprise SU8 photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

The cavity may have a height selected such that an operating voltage ofthe structure is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

Depositing the sacrificial layer may comprise evaporating a compositionthat comprises a solvent, and then depositing the composition as thesacrificial layer. At least 70% and no more than 90% of the solvent maybe evaporated.

According to another aspect, there is provided a method for fabricatinga layered structure, the method comprising: depositing a firstpolymer-based layer on a substrate assembly that functions as a bottomelectrode; patterning the first polymer-based layer to be a cavity;depositing a sacrificial layer on a separate substrate; depositing asecond polymer-based layer over the sacrificial layer; depositing a topelectrode on the second polymer-based layer; depositing a thirdpolymer-based layer on the electrode such that the top electrode isbetween the second and third polymer-based layers; adhering the firstand third polymer-based layers together such that the cavity is closedby the first and third polymer-based layers; and etching away thesacrificial layer such that the second polymer-based layer is releasedfrom the separate substrate.

The top electrode may be embedded within the second and thirdpolymer-based layers.

The method may further comprise cross-linking the first and thirdpolymer-based layers prior to adhering the first and third polymerlayers together.

Patterning the first polymer-based layer and etching away thesacrificial layer may be performed using organic and non-toxic solvents.

The first polymer-based layer may be photosensitive, and patterning thefirst polymer-based layer to be the cavity may comprise: cross-linking aportion of the first polymer-based layer to remain following the etchingby exposing the portion to ultraviolet radiation; and applying aphotoresist developer to etch uncrossed-linked areas of the firstpolymer-based layer.

The second polymer-based layer may be thicker than the thirdpolymer-based layer.

The second polymer-based layer may be at least five times thicker thanthe third polymer-based layer.

The relative thickness of the second polymer-based layer to the thirdpolymer-based layer may be selected such that the top electroderesonates at a frequency of at least 1 MHz.

The fabrication may be performed at a temperature of no more than 150°C.

The substrate assembly may be flexible and bonded to a rigid carrier.

The top electrode may comprise a conductive polymer.

The substrate assembly may comprise an optically-transparent material.

The substrate assembly may further comprise an optically-transparentconductive bottom electrode on the substrate.

Adhering the first and third polymer layers together may comprise:treating surfaces of the first and third polymer layers to be adhered toeach other with plasma; aligning the surfaces to each other; andpressing the surfaces together.

The adhering may be done in a bonding chamber at pressure of no morethan 0.001 Torr.

The method may further comprise, after the adhering, trapping charge inthe first polymer-based layer by: applying a voltage across the topelectrode and the substrate assembly such that a portion of the firstpolymer-based layer contacting the top electrode is pulled into contactwith the substrate assembly; maintaining the portion of the firstpolymer-based layer contacting the top electrode and the substrateassembly in contact for a period of time; and then ceasing applying thevoltage.

The sacrificial layer may comprise a polymer.

Depositing the second polymer-based layer on the sacrificial layer maycomprise completely covering the sacrificial layer with the secondpolymer-based layer.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

Following the adhering a watertight seal may be around the cavity.

The sacrificial layer may be non-reactive when exposed to the secondpolymer-based layer and to a photoresist developer used during thepatterning of the second polymer-based layer. The second polymer-basedlayer may be non-reactive when exposed to an etchant used to etch awaythe sacrificial layer.

The first, second, and third polymer-based layers may comprise SU8photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

The cavity may have a height selected such that an operating voltage ofthe transducer is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

Depositing the sacrificial layer may comprise evaporating a compositionthat comprises a solvent, and then depositing the composition as thesacrificial layer. At least 70% and no more than 90% of the solvent maybe evaporated.

According to another aspect, there is provided a capacitivemicromachined ultrasound transducer, comprising: a substrate assemblythat functions as a bottom electrode; a first polymer-based layersuspended above a sealed cavity between the first polymer-based layerand the substrate assembly; a second polymer-based layer placed on thefirst polymer-based layer; and a top electrode between the first andsecond polymer-based layers.

The top electrode may be embedded within the first and secondpolymer-based layers.

The second polymer layer may be thicker than the first polymer-basedlayer.

The second polymer-based layer may be at least five times thicker thanthe first polymer-based layer.

The cavity may have a height selected such that an operating voltage ofthe transducer is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

The cavity may be watertight.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

The sacrificial layer may be non-reactive when exposed to the first andsecond polymer-based layers and to a photoresist developer used duringthe patterning of the first and second polymer-based layers. The firstand second polymer-based layers may be non-reactive when exposed to anetchant used to etch away the sacrificial layer.

The first and second polymer-based layers may comprise SU8 photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

According to another aspect, there is provided a layered structure,comprising: a substrate assembly that functions as a bottom electrode; afirst polymer-based layer suspended above a closed cavity between thefirst polymer-based layer and the substrate assembly; a secondpolymer-based layer placed on the first polymer-based layer; and a topelectrode between the first and second polymer-based layers.

The top electrode may be embedded within the first and secondpolymer-based layers.

The second polymer layer may be thicker than the first polymer layer.

The second polymer layer may be at least five times thicker than thefirst polymer layer.

The cavity may have a height selected such that an operating voltage ofthe structure is no more than 50 Volts.

The cavity may have a height of no more than 0.3 μm.

The cavity may be watertight.

The substrate assembly may comprise a conductive substrate.

The substrate assembly may comprise a non-conductive substrate and aconductive bottom electrode on the substrate.

The sacrificial layer may be non-reactive when exposed to the secondpolymer-based layer and to a photoresist developer used during thepatterning of the second polymer-based layer. The second polymer-basedlayer may be non-reactive when exposed to an etchant used to etch awaythe sacrificial layer.

The first and second polymer-based layers may comprise SU8 photoresist.

The sacrificial layer may comprise an OmniCoat™ composition.

This summary does not necessarily describe the entire scope of allaspects. Other aspects, features and advantages will be apparent tothose of ordinary skill in the art upon review of the followingdescription of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more exampleembodiments:

FIGS. 1-15 are schematic diagrams sequentially arranged for illustratingoperations comprising a method for fabricating a polymer-based CMUT,according to one example embodiment.

FIG. 16 depicts a schematic diagram of a CMUT in contrast with theembodiment depicted in FIGS. 1-15.

FIGS. 17 and 18 depict schematic diagrams of curved polymer-based CMUTs,according to additional example embodiments.

FIGS. 19-30 are schematic diagrams sequentially arranged forillustrating operations comprising a method for fabricating apolymer-based CMUT, according to another example embodiment.

FIG. 31 depicts a schematic diagram of a polymer-based CMUT subject tocharge trapping effects, according to another example embodiment.

FIGS. 32-37 depict perspective views sequentially arranged forillustrating operations comprising a method for fabricating apolymer-based CMUT, according to another example embodiment.

FIGS. 38-44 depict experimental data relating to CMUTs fabricatedaccording to the example embodiment of FIGS. 1-15.

DETAILED DESCRIPTION

In an ultrasound imaging system, ultrasonic waves emitted by atransducer travel along soft tissues, creating wave reflections (echoes)at the interfaces between tissues with different densities (e.g., fatand muscle); these echoes travel back to the transducer and arecollected and processed to form an ultrasound image. The collection andmanipulation of multiple echo signals along different directions is thebasis of ultrasound image formation. Ultrasound transducers are a keycomponent in an ultrasound imaging system, which transform electricalvoltage into acoustic waves and vice versa.

Medical ultrasound systems have traditionally used piezoelectricmaterials for their transducers since the 1930s. Materials such aspiezoelectric crystals (e.g., quartz), ceramics (e.g., lead zirconatetitanate (PZT)), and thermoplastic fluoropolymers (e.g., polyvinylidenefluoride (PVDF)) have been used as the transducer materials. Despite thefact that piezoelectric transducers technology is mature, it suffersfrom many drawbacks such as the technical challenges in fabricatinglarge two-dimensional arrays due to interconnect technologies andintegration with electronics at the die-level.

Acoustic impedance (i.e., speed of sound in a material multiplied by itsdensity, units: Rayls) is a measure of the opposition that a systempresents to the acoustic flow resulting from an acoustic pressureapplied to the system. It is an important figure in piezoelectric-basedultrasound systems since it determines their acoustic efficiency, whichrepresents how much of the acoustic power is effectively transferred totissues. An acoustic matching layer is typically used in biomedicalpiezoelectric-based systems to reduce the impedance mismatch between thecrystals and tissues (30 MRayls to 1.5 MRayls); otherwise just afraction of the acoustic power could be used. These matching layers aretypically made of high-density rubber combined with liquid gel and arelocated between the crystals and the body.

Capacitive micromachined ultrasound transducers (CMUTs) are analternative technology to conventional piezoelectric-based transducers.A CMUT may be modeled as a parallel-plate capacitor with a fixedelectrode at bottom, a suspended membrane over a closed cavity, andanother electrode patterned on top of the cavity. Ultrasound waves aregenerated when an AC signal superimposed on a DC voltage is appliedbetween both electrodes; conversely ultrasound waves can be detected bymeasuring the variation in capacitance of the device while a DC voltageis applied in the presence of incoming ultrasound waves. The effectivedistance (i.e., thickness of the cavity and membrane) is preferably assmall as possible for two reasons: 1) in order to maintain a low (e.g.,less than 150 V) operating voltage during transmission (i.e., ultrasoundwaves generated from CMUT) and 2) to maintain a good sensitivity duringreception (i.e., ultrasound waves arriving to the CMUT), since thecapacitance variation is greater for large capacitance devices (i.e.,those devices with comparatively thin dielectrics).

Silicon nitride and polysilicon are the most popular materials formembranes in conventional CMUTs fabrication, while metals such asaluminum or chromium are patterned on top of the membranes to become thetop electrode. The membrane materials are chosen mainly because of theirmechanical properties so the membranes can be as thin as possible inorder to minimize the effective gap between the bottom and top (or“hot”) electrodes.

By decreasing the effective gap between electrodes, the electric-fieldshare of the gap and the capacitance increase, and the impedancematching to the electronics improves. Starting with a desiredoperational frequency and a specific limit for the biasing voltage, theCMUT membrane should be designed to be as thick as possible given thefact that its bandwidth linearly increases with its thickness.

In contrast to the above materials, photopolymers are inexpensive andcan be patterned using UV light; their low density and high mechanicalstrength make the application of these polymers interesting in theultrasound field mainly because acoustic impedance matching with themedium into which ultrasonic waves are sent and received can be greatlyimproved. Nonetheless the challenge in fabricating CMUTs using polymersis that a thick membrane with a metal electrode on top is needed toreach the MHz region, contravening the required short gap betweenelectrodes for low operational voltages and maximum sensitivity. Therehas been some research in fabricating CMUTs using polymer materials;however, given their large membrane thickness the operational voltageswere in the order of hundreds of volts, which is incompatible withbiomedical ultrasound applications. Moreover, the mentioned devices aresuitable operating in air only, and not for operating in conjunctionwith human tissues.

The embodiments described herein are directed at methods for fabricatinga layered structure, such as a CMUT, and at that structure itself. In atleast some example embodiments, surface micromachining may be used tofabricate the layered structure. When surface micromachining is used, asacrificial layer is deposited on a substrate; the sacrificial layer ispatterned into a first shape; a first polymer-based layer is depositedon the sacrificial layer; an electrode is deposited, on the firstpolymer-based layer, above the sacrificial layer; a second polymer-basedlayer is deposited on the electrode such that the electrode is between,and in some embodiments embedded, within the first and secondpolymer-based layers; and the sacrificial layer is then etched away toform a cavity under the electrode.

In at least some example embodiments in which surface micromachining isused to manufacture a CMUT, the sacrificial layer is deposited on to asubstrate assembly that functions as a bottom electrode; the sacrificiallayer is patterned to be shaped as a cavity of the CMUT; the firstpolymer-based layer is deposited on the sacrificial layer; a via hole ispatterned through the first polymer-based layer to the sacrificiallayer; a top electrode is patterned, above the sacrificial layer, on thefirst polymer-based layer; a second polymer-based layer is deposited onthe top electrode such that the top electrode is embedded within thefirst and second polymer-based layers; the sacrificial layer is etchedaway, using the via hole, to form the cavity of the CMUT; and the cavityis closed.

In at least some different embodiments, wafer bonding may be used tofabricate the layered structure. When wafer bonding is used, a firstpolymer-based layer is deposited on a first substrate; the firstpolymer-based layer is patterned to be a cavity; a sacrificial layer isdeposited on a second substrate; a second polymer-based layer isdeposited over the sacrificial layer; an electrode is deposited on thesecond polymer-based layer; a third polymer-based layer is deposited onthe electrode such that the electrode is between, and in someembodiments embedded, within the second and third polymer-based layers;the second and third polymer-based layers are cross-linked; the firstand third polymer-based layers are adhered together such that the cavityis sealed by those layers; and the sacrificial layer is etched away suchthat the second polymer-based layer is released from the secondsubstrate.

In at least some example embodiments in which wafer bonding is used tomanufacture a CMUT, the first polymer-based layer is deposited on thesubstrate assembly, which functions as the bottom electrode; the firstpolymer-based layer is patterned to be a cavity of the CMUT; thesacrificial layer is deposited on a separate substrate; the secondpolymer-based layer is deposited over the sacrificial layer; the topelectrode is deposited on the second polymer-based layer; the thirdpolymer-based layer is deposited on the top electrode such that the topelectrode is embedded within the second and third polymer-based layers;the first and third polymer-based layers are adhered together such thatthe cavity is closed; and the sacrificial layer is etched away such thatthe second polymer-based layer is released from the separate substrate.

As used herein, “embedding” an electrode with a polymer means completelycovering the electrode with the polymer, except for any electricalconnections made with that electrode.

Also as used herein, “patterning” a material means to selectively removethat material either directly (e.g., if it is photosensitive) or byusing a masking layer (e.g., in the case of the OmniCoat™ composition,as discussed further below).

In at least some of the embodiments in which a polymer-based CMUT isfabricated, the polymer material may be inexpensive, easy to process,and be capable of being made in large arrays. Additionally, in contrastto conventional CMUTs, the top electrode is embedded within two polymerlayers, with the bottom layer being thinner than the top layer; this,combined with forming a sufficiently thin CMUT cavity by etching away asacrificial layer, permits the CMUT to reach the MHz operative regionwithout requiring unacceptably high operating voltages.

A detailed description of the fabrication operations and relevantinformation about the materials used follows. FIG. 1 to FIG. 18 areschematic diagrams sequentially arranged for illustrating operationscomprising a method for fabricating a polymer-based CMUT according to asurface micromachining embodiment. FIGS. 19-30 depict an analogousmethod according to a wafer bonding embodiment.

Surface Micromachining

Referring now to FIG. 1, there is shown a cross-sectional view of asubstrate assembly in the form of an electrically conductive substrate10 (an electrically conductive silicon wafer in this case). A lowelectrical resistance of this substrate 10 facilitates the substrate 10acting as a bottom electrode in the finished CMUT. In some differentembodiments (not depicted), a dedicated bottom electrode can bepatterned over an insulating substrate 10 as an alternative. CMUTs aretypically fabricated using silicon wafers as substrates, but any versionof the rigid to semi-rigid surface with a smooth hydrophilic surface issufficient to be used with this methodology. The surface of thesubstrate 10 is hydrophilic in at least some example embodiments inorder to achieve a wet release of the final membrane described insubsequent operations.

A sacrificial layer 11 is deposited on the substrate 10 by spin coating.This sacrificial layer 11 will become the evacuated cavity 21 in thefinished CMUT. The required thickness of the sacrificial layer 11 for aCMUT can range from a few hundreds of nanometers (nm) (e.g., 300 nm) toa couple of micrometers (μm) (e.g., 2 μm). A highly selective etchant isused to etch away the sacrificial layer 11 without damaging the CMUT'smembranes, which are formed in subsequent operations as described below.

The OmniCoat™ composition by MicroChem Corp. has an excellentselectivity during etching and it enhances the adhesion of photoresiststo different substrates. The two main chemical components in OmniCoat™composition are cyclopentanone (a solvent that gets evaporated) andPropylene Glycol Monomethyl Ether (PGME). The OmniCoat™ composition alsocomprises a polymer (less than 1% of total volume) and a surfactant(also less than 1% of total volume). The OmniCoat™ composition is notphotosensitive and its typical thickness during spin coating ranges from5 nm to 15 nm, which limits its conventional use to releasing largestructures by immersing them in developer for a few hours until thestructures get released and float away from the carrying substrate.Still, this thickness range (5-15 nm) is well below the typicalthickness used for sacrificial layers when conventional CMUTs arefabricated (200 nm-5,000 nm). In the depicted example embodiments, theOmniCoat™ composition is used for the sacrificial layer 11. In at leastsome example embodiments, the OmniCoat™ composition is evaporated priorto depositing it as the sacrificial layer 11. For example, evaporating acertain percentage of the solvents of the off-the-shelf OmniCoat™composition (e.g., 85%) prior to its deposition allows a relativelythick sacrificial layer 11 (e.g., 0.3 μm), to be deposited in a singlestep. This may help to increase efficiency and to allow for greaterprecision in laying a sacrificial layer 11 of a desired thickness. In atleast some other embodiments (not depicted), the OmniCoat™ compositionmay not be evaporated at all prior to its deposition as the sacrificiallayer 11; for example, without any pre-deposition evaporation, multiplelayers of the OmniCoat™ composition may be deposited in order to reach adesired thickness. In further additional embodiments (not depicted),while a certain proportion of its solvents may be evaporated, thatproportion may be more or less than 85%. For example, if a thinnersacrificial layer 11 is desired, then a smaller percentage (e.g., 70%)of the solvents in the OmniCoat™ composition may be evaporated;alternatively, more (e.g., 90%) of the solvents may be evaporated. Moregenerally, this pre-deposition evaporation may be performed on whatevercomposition is used as the sacrificial layer 11.

The sacrificial layer 11 is patterned to create the areas that willbecome the cavity 21 (shown in FIG. 15) in the final device as well asreleasing channels to permit access to this cavity 21. Given the factthat the OmniCoat™ composition is not photosensitive it cannot bedirectly patterned and needs to be removed indirectly.

Referring now to FIG. 2, a layer of positive photoresist (PR) 12 isdeposited on top of the sacrificial layer 11. This photoresist (S1813)is selected so that its developer dissolves the cross-linked photoresist13 (shown in FIG. 3), which results after ultraviolet light (UV)exposure as well as the sacrificial material 11.

Referring now to FIG. 3, the photoresist layer 12 is exposed to UV usinga photomask and a mask aligner. The areas exposed to UV becomecross-linked photoresist 13 and the areas not exposed to UV are leftintact (uncross-linked).

Referring now to FIG. 4, the cross-linked photoresist 13 is etched away(removed) by placing the sample in an aqueous solution containing analkaline-based photoresist developer (MF319). The photoresist 12 that isuncross-linked remains intact.

Referring now to FIG. 5, while the sample is still in the photoresistdeveloper (MF319) from FIG. 4, the etching continues and startsdissolving the sacrificial layer 11. The patterned photoresist layer 12acts as a masking layer to protect the sacrificial layer 11 underneath.

The etching is stopped as soon as the sacrificial layer 11 under thecross-linked photoresist 13 is removed, leaving the substrate 10 exposedfor subsequent operations.

Referring now to FIG. 6, the masking layer of positive photoresist 12 isremoved by immersing the sample in acetone or any other solvent suitableto dissolve the positive PR 12 without damaging the sacrificial layer11. The sacrificial layer 11 offers an excellent selectivity (chemicalresistance) to the solvent used (acetone). What is left behind is apatterned sacrificial layer 11 containing the areas that will become thecavity 21 in the final device as well as the etch channels.

Referring now to FIG. 7, a first polymer-based layer 14 comprising anegative photosensitive polymer-based material (SU8 photoresist,hereinafter interchangeably referred to simply as “SU8”) 14 isdeposited, conformally covering the sacrificial layer 11. The thicknessof the layer 14 is designed to be as thin as possible to conformallycoat the sacrificial layer 11 and to be able to maintain good electricalinsulation between the conductive substrate 10, which acts at thefinished CMUT's bottom electrode, and a top electrode 16 (shown in FIGS.10-15). The SU8 comprises Bisphenol A Novolac epoxy dissolved in anorganic solvent, and comprises up to 10 wt %Triarylsulfonium/hexafluoroantimonate salt; in different exampleembodiments (not depicted), the polymer-based material may have adifferent composition. The SU8 is also optically transparent, whichfacilitates inspection of the finished device. In at least somedifferent embodiments, a material may be used in place of the SU8, andthat replacement material may be non-opaque (i.e., partially or entirelytransparent).

The layer 14 in at least the depicted example embodiment comprises aphotopolymer. Photopolymers are inexpensive and can be patterned usingUV; their low density and high mechanical strength make the applicationof these polymers interesting in the ultrasound field mainly because theimpedance matching with the medium into which ultrasonic waves aretransmitted and from which reflected waves are received can be greatlyimproved. Nonetheless the challenge in fabricating CMUTs using polymersis that, conventionally, a thick membrane with a metal electrode on topis needed to reach the MHz operational region, contravening the requiredshort gap between electrodes that facilitate low operational voltagesand maximum sensitivity.

Referring now to FIG. 8, the layer 14 is exposed to UV using a photomaskand a mask aligner. The areas exposed to UV light become cross-linkedareas 15 of the layer 14 and the areas not exposed to UV are left asuncross-linked areas.

Referring now to FIG. 9, the uncross-linked areas of the firstpolymer-based layer 14 are etched away (removed) by placing the samplein an aqueous solution containing a negative photoresist developer (SU8developer). The cross-linked areas 15 remain intact.

Referring now to FIG. 10, an electrically conductive top electrode(chromium) 16 is patterned on top of the cross-linked areas 15 of thefirst polymer-based layer 14 using lift-off methods. This electrode 16is in at least some example embodiments made as thin as possible withoutsacrificing electrical conductivity in order to not greatly modify thestructural properties of the cross-linked areas 15, which will laterbecome the membrane of the finished device.

The material for this electrode 16 in at least the depicted exampleembodiment is typically metallic; nevertheless any other materialcapable of fulfilling the functions of the top electrode 16 can be used(e.g., conductive polymers, optically transparent materials, etc.). Agood adhesion between this top electrode 16 and the cross-linked areas15 is present in order to avoid any potential delamination during normaloperation of the finished device. Using chromium as the top electrode 16when the cross-linked areas 15 comprise SU8 may help to facilitateadhesion.

At this point the overall thickness of the membrane (i.e., thecross-linked areas 15 and the top electrode 16) is thin compared to itsdiameter so that its resonant frequency would be just a fraction of thedesired operational frequency in the finished device. A much thickermembrane is required in order to reach the desired operational frequencyin, for example, the MHz range.

Referring now to FIG. 11, a second polymer-based layer 17 is depositedover the membrane, conformally coating the sacrificial layer 11, thefirst polymer-based layer 15, and the top electrode 16. The secondpolymer-based layer 17 is of the same photosensitive polymer (SU8) asthe first polymer-based layer 14 in at least the depicted exampleembodiment; however, in different embodiments (not depicted) the layers14,17 may comprise different polymers. The thickness of this secondpolymer-based layer 17 is designed to be a few times (˜5) thicker thanthat of the cross-linked areas 15 of the first polymer-based layer 14.

Referring now to FIG. 12, following the same process as described inrespect of FIG. 8, the second polymer-based layer 17 is exposed to UVusing a photomask and a mask aligner. The areas exposed to UV lightbecome cross-linked areas 18 and the areas not exposed to UV are leftintact (uncross-linked).

Referring now to FIG. 13, the uncross-linked areas of the secondpolymer-based layer 17 are etched away (removed) by placing the samplein an aqueous solution containing a negative photoresist developer (SU8developer). The cross-linked areas 18 remain intact.

At this point, the top electrode 16 becomes embedded between thecross-linked areas 15,18 of the two polymer-based layers 14,17. Theadvantage of this approach is that the membrane is still able to operatein the MHz region because of the added thickness from the secondpolymer-based layer 17, which increases the effective stiffness whilestill maintaining a low operational voltage thanks to the smalleffective distance between the bottom substrate 10 and the embedded topelectrode 16.

This fabrication process is not limited to the operation in the MHzrange for biomedical ultrasound imaging. If desired, the same or ananalogous fabrication process can be used to obtain membranes thatoperate in the Hz and kHz region for air-coupled operation applications,for example. The final operational frequency of the membrane depends onthe geometry of the cell. This means that membranes that resonate atdifferent frequencies can be operated with very similar voltages. Forexample, two membranes with the same diameter can operate with the samevoltage (same effective distance between electrodes), but one can bethinner for lower frequencies and the other ticker for high-frequencyoperation.

Referring now to FIG. 14, the sample is then immersed in an aqueousalkaline-based solution containing the etchant (MF319) of thesacrificial layer 11. The patterned sacrificial layer 11 is graduallyremoved though via holes and etch channels until it is fully dissolved.At this point the etchant is replaced by water and then by isopropanol(IPA). A critical point dryer system is used to release the membrane,avoiding stiction problems and remaining with a membrane suspended abovean un-sealed cavity 19.

Referring now to FIG. 15, the sample is encapsulated by a bio-compatiblematerial 20 (a poly(p-xylylene) polymer, such as parylene) inside alow-pressure chamber; while various pressures may be used, in thedepicted example embodiment the pressure within the chamber is 1×10⁻³Torr. The encapsulating material 20 conformally coats the entire sample,sealing the via holes and etch channels to form a closed cavity 21. Inat least the depicted example embodiment, the cavity 21 is vacuum sealedand is watertight and airtight, which helps to avoid squeeze-filmeffects and to reduce the risk of voltage breakdown. In differentexample embodiments, the cavity 21 may not be vacuum sealed, or may bewatertight and not airtight.

At this point the fabrication process is complete and the finisheddevice (i.e., the CMUT) results. Any electrical interconnection is madebefore this step as the biocompatible material (parylene) is anexcellent electrical insulator and is safe for use on humans.

This fabrication process may use optically transparent orsemi-transparent materials for any one or more of the substrate (e.g.glass or quartz), for the electrodes (e.g. Indium oxide, which issemi-transparent) and for the sealing layer (parylene). This leads to anoptically transparent or semi-transparent transducer.

In the CMUT depicted in FIG. 15, the cross-linked areas 15 of the firstpolymer-based layer 14 is significantly thinner than the cross-linkedareas 18 of the second polymer-based layer 17. FIG. 16 depicts anexample CMUT in which the electrode 16 is located above the secondpolymer-based layer 18 and then encapsulated by the encapsulatingmaterial 20.

The operating voltage of the CMUT of FIG. 15 is much lower than that ofFIG. 16. For instance, the resonant frequency of the membranes in FIG.15 and FIG. 16 is the same since all the materials and thickness remainthe same except for the location of the top electrode. The operationalvoltage of the CMUT shown in FIG. 15 is 50 Volts, whereas theoperational voltage of the CMUT shown in FIG. 16 is 300 Volts, which isprohibitive in medical ultrasound systems.

The described materials and fabrication process of FIGS. 1-15 may beused to fabricate the CMUT on flexible substrates. This is an advantagefor conformal imaging systems in which the ultrasound elements are to becurved around different parts of the human body as depicted in FIG. 17and FIG. 18. The polymer materials used for fabrication are sufficientlyflexible, allowing the CMUT to bend around small radii of curvaturewithout sacrificing performance or mechanical stability.

Traditional CMUTs fabricated with polysilicon and silicon nitride aregenerally inflexible and employ hazardous chemicals (potassium hydroxideand hydrochloric acid) during etching. The chemicals may present a riskfor people working with these materials as they are corrosive and thevapours can cause internal organ damage, resulting in severe and in somecases fatal consequences. The fabrication operations according to atleast some of the example embodiments herein can be performed in simplelow-cost and safe fabrication facility.

The aforementioned fabrication process in at least some exampleembodiments employ non-hazardous materials, i.e. only organic solventsare used during fabrication (acetone, isopropanol, SU8 developer, andpositive photoresist developer). The health risks associated with anaccidental prolonged exposure to these materials are generally limitedto drowsiness and minor skin irritation. The etchant used to remove theOmniCoat™ composition (MF319 or Tetramethylammonium hydroxide diluted inwater) can be safely disposed of in ordinary laboratory drain systemswhen diluted in water as it is considered a mild base.

The fabrication costs associated with the fabrication process depictedin FIGS. 1-15 is significantly less than the cost required tomanufacture conventional designs. As of December, 2017, the estimatedmaterial costs to fabricate an array of ultrasound transducers is lessthan US$100 inside a university laboratory, with a potential costreduction if mass produced; meaning that the fabricated devices can beconsidered at some point disposable.

The maximum temperature required to manufacture CMUTs using thedescribed process in FIGS. 1-15 for this process is 150° C.,consequently requiring minimal thermal protection systems and usingminimal thermal budget compared to conventional fabrication processesusing polysilicon.

Additionally, using polymers as structural material for CMUTs means thatif an acoustic matching layer is required it can be manufactured usingthe same kind of polymer materials with embedded fillers.

Referring now to FIGS. 32-37, there are depicted perspective viewssequentially arranged for illustrating operations comprising a methodfor fabricating a polymer-based CMUT, according to another exampleembodiment.

Referring now to FIG. 32, the electrically-conductive substrate 10(e.g., silicon wafer) is uniformly coated with a sacrificial material(e.g., the OmniCoat™ composition) and baked to form the sacrificiallayer 11. A layer of positive photoresist (S1813) is deposited on top ofthe OmniCoat™ composition and baked. The sample is selectively exposedto UV to pattern the sacrificial layer 11's design. The sample isimmersed in positive photoresist developer (MF319). The developerdissolves both unexposed areas in S1813 and the OmniCoat™ compositionunderneath, thereby leaving a patterned design of the sacrificial layer11. The sacrificial layer 11 comprises an area to eventually form theCMUT cavity 21 as well as etch channels 37 and etch via holes 35.

Referring now to FIG. 33, the sample coated with the first polymer-basedlayer 14 comprising a polymer-based material (SU8), conformally coveringthe sacrificial layer 11. The thickness of the layer 14 is by designselected to be as thin as possible as long as the sacrificial layer 14is covered and the breakdown voltage of the polymer exceeds the desiredoperational voltage. The sample is exposed to UV to pattern the anchorpoints of the sample as well as the first layer of the membrane. Thesample is baked and developed in SU8 developer, leaving open windows forthe etch channels 37.

Referring now to FIG. 34, the electrically conductive electrode 16(chromium) is patterned on top of the first polymer-based layer 14 usinglift-off micromachining methods; electrical connections 39 to theelectrode 16 are concurrently patterned. The thickness of the electrode16 is as thin as possible as long as a low resistance path ismaintained. In at least some different embodiments (not depicted), theelectrode 16 may comprise non-metallic materials, such as one or moreconductive polymers.

Referring now to FIG. 35, the second polymer-based layer 17 isconformally coated on top of the electrode 16, covering the stackcomprising the sacrificial layer 11, first polymer-based layer 14, andmetal electrode 16. The second polymer-based layer 17 also comprises theSU8. The sample is exposed to UV to pattern the CMUT membrane and leaveopen areas for the via holes 35 on the first polymer-based layer 14. Thepurpose of this second polymer-based layer 17 is to increase theeffective thickness of the membrane and therefore increase its resonantfrequency. Electrical contacts are exposed to air. In the depictedexample embodiment, only the areas corresponding to where the cavity 21will be located in the finished CMUT is patterned with the secondpolymer-based layer 17; in different embodiments (not depicted), morethan these areas may be patterned with the second polymer-based layer17.

Referring now to FIG. 36, the sample is immersed in positive photoresistdeveloper (MF319, same etching chemical as for the OmniCoat™composition). Developer removes the sacrificial material through the viaholes 35 and etch channels 37. The developer (MF319) is replaced bywater and then by isopropyl alcohol (IPA) in a wet environment. Thesample is immersed in IPA inside a critical point dryer system torelease the membrane. Liquid CO₂ replaces IPA in a high-pressureenvironment and then the liquid CO₂ is converted to gaseous CO₂. At thispoint the membrane is suspended on the cavity 19. While a critical pointsystem is used in the fabrication depicted in FIGS. 32-37, in differentembodiments (not depicted) it may be omitted, particularly if the CMUTmembrane is not prone to stiction given its dimensions.

Referring now to FIG. 37, the sample is placed in a low-pressure chamber(e.g., operated at 1×10⁻³ Torr) and conformally coated with theencapsulating material 20 comprising polymer materials (parylene) sothat the cavity 21 is vacuum-sealed (i.e., airtight) and watertight. Theparylene's thickness is selected so the mechanical properties of theencapsulating material 20 in the finished CMUT are very similar (and insome embodiments identical) to the SU8 (e.g., in terms of density andYoung's modulus); accordingly, in at least some embodiments thecollective thickness of the encapsulating material 20 and the secondpolymer-based layer 17 are considered when comparing that thickness tothat of the first polymer-based layer 15, the ratio of which influencesthe finished CMUT's operational frequency. The encapsulating material 20seals the via holes 35 and etch channels 37, leaving a vacuum-sealed andwatertight cavity once the sample is removed from the low-pressurechamber. Areas for electrical interconnections are protected prior thissealing step.

The resulting finished CMUT is a sealed CMUT element with a low pull-involtage given its small effective separation between electrodes. Thecarrying substrate need not be limited to rigid materials; flexiblematerial temporarily attached to a rigid carrier may be used as well.The sample may be electrically interconnected to an interface circuitprior to sealing (FIG. 36). An acceptable adhesion between polymermaterial and electrode is used to avoid mechanical failure duringoperation.

Wafer Bonding

In at least some example embodiments, wafer bonding technology can beused to manufacture a similar version of the CMUT depicted in FIG. 15.In this approach, the materials are deposited and processed in twoseparate substrates, such as silicon wafers. The materials deposited onthe separate substrates are then adhered together and further processedto obtain CMUTs. The detailed fabrication description follows.

Referring now to FIG. 19, the first polymer-based layer 14 comprising apolymer-based material (SU8) is deposited on top of a substrate assemblycomprising the bottom substrate 10, which acts as the bottom electrodein the finished CMUT and which in the depicted example embodiment iselectrically conductive.

Referring now to FIG. 20, the first polymer-based layer 14 is exposed toUV using a photomask. The areas exposed to UV become the cross-linkedareas 15 and the areas not exposed to UV are left uncross-linked.

Referring now to FIG. 21, the uncross-linked areas of the firstpolymer-based layer 14 are etched away (removed) using photoresistdeveloper (SU8 developer). The cross-linked areas 15 remain intact. Thecross-linked areas 15 that remain following etching will act as pillarssupporting the CMUT membranes.

Referring now to FIG. 22, in a separate substrate 30 (silicon wafer orany other rigid and smooth substrate) a sacrificial layer 11 isdeposited on top by spin coating. This sacrificial layer 11 will be usedto release the separate substrate 30 following adhering, as discussedfurther below.

Referring now to FIG. 23, the second polymer-based layer 17, which inthe depicted example embodiment comprises the same photosensitivepolymer (SU8) as used for the first polymer-based layer 14, is depositedon top of the sacrificial layer 11; this layer 17 will become the toppart of the finished CMUT.

Referring now to FIG. 24, the second polymer-based layer 17 is exposedto UV using a photomask and a mask aligner. The areas exposed to UVbecome the cross-linked areas 18.

Referring now to FIG. 25, the electrically conductive top electrode 16(chromium) is patterned on top of the cross-linked areas 18 usinglift-off methods.

Referring now to FIG. 26, a third polymer-based layer 32 of the samephotosensitive polymer (SU8) comprising the first and secondpolymer-based layers 14,17 is deposited on the top electrodes 16,conformally coating the metal electrodes 16 and the cross-linked areas18 of the second polymer-based layer 17. At this point, the topelectrode 16 becomes encapsulated between the cross-linked areas 18 ofthe second polymer-based layer 17 and the third polymer-based layer 32.

Referring now to FIG. 27, the third polymer-based layer 32 is exposed toUV using a photomask and a mask aligner. The areas exposed to UV becomecross-linked areas 33. The purpose of the cross-linked areas 33 is toact as dielectric layer between the two electrodes (the bottom substrate10 and the top electrode 16) in the finished CMUT. Cross-linking thefirst and third polymer-based layers 14,32 serves to promote adhesionbetween those layers; in at least some different example embodiments(not depicted), the cross-linking may be skipped if the layers 14,32 canbe suitably adhered to each other without cross-linking.

Referring now to FIG. 28, the surfaces of the separate samples as shownin FIG. 21 and FIG. 27 are treated with oxygen plasma, which allows thesurfaces of both samples to be permanently adhered. The samples arealigned and placed face to face in a vacuum environment and pressedagainst each other. The vacuum may be any suitable pressure, such as1×10⁻³ Torr as described above in the surface micromachining embodiment.In different embodiments (not depicted), the samples may not be treatedwith oxygen plasma.

Referring now to FIG. 29, after releasing the pressure both samples arenow permanently attached, creating an array of vacuum-sealed cavities21.

Referring now to FIG. 30, the sample is immersed in an aqueousalkaline-based solution containing the etchant (MF319) of thesacrificial layer 11. The sacrificial layer 11 is gradually removeduntil the separate substrate 30 is released. This etchant does notattack the polymer not the metallic materials used. At this point thefabrication process is complete and the device becomes watertight. Aswith the surface micromachining embodiment of FIGS. 1-15, in certainembodiments the cavities 21, while closed, may not be airtight orwatertight; in other embodiments, the cavities 21 may be watertight andnot airtight.

The CMUT fabricated using wafer bonding does not comprise theencapsulating material 20 in the depicted example embodiments as thecavities 21 are vacuum-sealed following adhesion. This simplifiesfabrication.

The fill factor (number of CMUTs per unit area) may be improved usingwafer bonding vs. surface micromachining, as the CMUTs can be placedcloser to each another since the releasing holes (vias) and channels donot exist. By using hexagonal or square membranes the fill factor can beincreased, relative to circular membranes.

In at least some example embodiments, Roll-to-Roll (R2R) technology maybe applied to fabricate the cavity 21.

In the wafer bonding embodiment, one or both of the substrates 10,30 maybe flexible if bonded to a rigid carrier.

Charge Trapping

Charge trapping effects in CMUTs may be observed, for example, when azero-bias resonator is fabricated by purposely trapping electricalcharges in a dielectric layer by applying a large bias voltage beyondpull-in. More generally, charge trappings effects may be observed forany resonator (including a CMUT) or layered device fabricated accordingto the embodiments described herein, including those that are notzero-bias. In the examples described herein, the trapped charging effectcontributes positive to the normal operation of the resonator (e.g., amaterially lower operational voltage may be used when trapped chargesare present).

Referring now to FIG. 31, electrical charges get trapped in the SU8membrane underlying the top electrode 16 of the CMUT of, for example,FIG. 15 or 30, when a DC voltage larger than pull in (VPI=65V) isapplied between the top electrode 16 and the bottom substrate 10, whichacts as the bottom electrode. This causes the membrane to collapse(e.g., to be pulled into contact with the substrate 10), resulting inthe electrical field acting on the membrane to increase. In at leastsome different example embodiments (not depicted) the cavity 21 issufficiently tall that the membrane does not contact the bottomsubstrate 10 when the DC voltage is applied. After removing the DCvoltage, the membrane returns to its initial position having electricalcharges 22 trapped in the dielectric film (SU8).

The electrical charges 22 trapped in the membrane contribute to theelectrostatic force during operation (acting like a built-in voltage),meaning that a lower DC bias voltage may be used to bring the membranecloser to the bottom substrate 10.

It has been experimentally shown that the electrical charges 22 gettrapped in the volume of the SU8 film (in theory, by the molecules'dipole alignment) and not on the metal electrode 16 (as an ordinarycapacitor). This prevents the CMUT from getting “discharged” even if itsterminals are shorted.

Experimental Results

Using the surface micromachining embodiment described in respect ofFIGS. 1-15, a set of linear arrays containing 64 and 128 CMUT elements,with each element comprising an interconnected matrix of CMUT cellssharing a common bottom electrode in the form of a conductive substrate10 (a silicon wafer), were fabricated. These CMUT elements are shown inFIG. 38 (64 elements) and FIG. 39 (128 elements). The total fabricationtime was 16 Hrs.

Detailed views of the fabricated arrays are shown in FIG. 40 and FIG. 41for a 64 and 128 element array, respectively. The diameter of the CMUTcells is 100 μm and 90 μm for the 64 and 128 element arrays,respectively. The thickness of the CMUT membranes is 7.31 μm, whichincludes the top electrode 16 and the cross-linked areas 15 of the firstpolymer-based layer 14 and the vacuum-filled cavity 21 has a height of0.3 μm. Electrical connections for external interface are located ateach end on the elements.

Acoustic measurements were performed in an oil bath using apiezoelectric transducer to validate the operation of the fabricatedpolymer CMUTs. The measured response is shown in FIG. 42, showing ashort pulse characteristic of ultrasound transducers. The frequencyspectrum (FFT) of the measured pulse is shown in FIG. 43, having afractional bandwidth of 111%.

Preliminary results show that it is possible to measure ultrasoundpulses using a polymer CMUT element as a passive receiver (i.e., no DCbias voltage applied). FIG. 44 shows the measurement of an ultrasoundpulse generated by a piezoelectric crystal located above a polymer CMUTelement in a liquid medium operating at an acoustic pressure typical formedical ultrasound imaging. The terminals of the CMUT (top and bottomelectrode) were directly connected to an oscilloscope.

The amplitude of the received signal when the CMUTs are operated as apassive device (no DC bias voltage) was 264 mVpp; this represents muchmore than the expected voltage obtained from typical piezoelectric-basedtransducers, in which the expected generated voltage across theterminals ranges between a few microvolts and 100 mV. The amplitude ofthe received signals was increased even further to almost 500 mVpp whena bias voltage of 15V was applied.

This implies that ultrasound signals can be directly processed withoutthe need of low-noise and high-gain amplifiers used in commercialpiezoelectric-based ultrasound systems, potentially reducing thephysical volume and weight in ultrasound probes and marking a stepforward towards a lightweight, low-power conformal ultrasound system.

In at least some embodiments, no acoustic matching layer is required tocouple the fabricated CMUTs (regardless of whether fabricated usingsurface micromachining or wafer bonding) to an aqueous medium. Thiscontrasts with the mandatory acoustic matching layer in conventionalpiezoelectric-based ultrasound imaging systems.

Additionally, in at least some example embodiments, one or both of thesurface micromachining and wafer bonding embodiments may furthercomprise an annealing operation. When SU8 is used during fabrication,annealing may be done at, for example, 150° C. for five minutes toanneal any cracks that may have formed during development.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. Accordingly, asused herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises” and“comprising”, when used in this specification, specify the presence ofone or more stated features, integers, steps, operations, elements, andcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components, andgroups. Directional terms such as “top”, “bottom”, “upwards”,“downwards”, “vertically”, and “laterally” are used in the followingdescription for the purpose of providing relative reference only, andare not intended to suggest any limitations on how any article is to bepositioned during use, or to be mounted in an assembly or relative to anenvironment. Additionally, the term “couple” and variants of it such as“coupled”, “couples”, and “coupling” as used in this description areintended to include indirect and direct connections unless otherwiseindicated. For example, if a first device is coupled to a second device,that coupling may be through a direct connection or through an indirectconnection via other devices and connections. Similarly, if the firstdevice is communicatively coupled to the second device, communicationmay be through a direct connection or through an indirect connection viaother devices and connections.

It is contemplated that any part of any aspect or embodiment discussedin this specification can be implemented or combined with any part ofany other aspect or embodiment discussed in this specification.

One or more example embodiments have been described by way ofillustration only. This description is presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the form disclosed. It will be apparent to persons skilled inthe art that a number of variations and modifications can be madewithout departing from the scope of the claims.

The invention claimed is:
 1. A method for fabricating a layeredstructure, the method comprising: (a) depositing a first polymer-basedlayer on a substrate assembly that functions as a bottom electrode; (b)patterning the first polymer-based layer to be a cavity; (c) depositinga sacrificial layer on a separate substrate; (d) depositing a secondpolymer-based layer over the sacrificial layer; (e) depositing a topelectrode on the second polymer-based layer; (f) depositing a thirdpolymer-based layer on the top electrode such that the top electrode isbetween the second and third polymer-based layers; (g) adhering thefirst and third polymer-based layers together such that the cavity isclosed by the first and third polymer-based layers; and (h) etching awaythe sacrificial layer such that the second polymer-based layer isreleased from the separate substrate.
 2. The method of claim 1, whereinthe top electrode is embedded within the second and third polymer-basedlayers.
 3. The method of claim 1, further comprising cross-linking thefirst and third polymer-based layers prior to adhering the first andthird polymer layers together.
 4. The method of claim 1, whereinpatterning the first polymer-based layer and etching away thesacrificial layer are performed using organic and non-toxic solvents. 5.The method of claim 1, wherein the first polymer-based layer isphotosensitive, and wherein patterning the first polymer-based layer tobe the cavity comprises: (a) cross-linking a portion of the firstpolymer-based layer to remain following the etching by exposing theportion to ultraviolet radiation; and (b) applying a photoresistdeveloper to etch uncross-linked areas of the first polymer-based layer.6. The method of claim 1, wherein: (a) relative thickness of the secondpolymer-based layer to the first polymer-based layer is selected suchthat the top electrode resonates at a frequency of at least 1 MHz; or(b) the second polymer-based layer is at least five times thicker thanthe first polymer-based layer.
 7. The method of claim 1, wherein thefabrication of the layered structure is performed at a temperature of nomore than 150° C.
 8. The method of claim 1, wherein the substrateassembly is flexible and bonded to a rigid carrier.
 9. The method ofclaim 1, wherein the top electrode comprises a conductive polymer. 10.The method of claim 1 wherein adhering the first and third polymerlayers together comprises: (a) treating surfaces of the first and thirdpolymer layers to be adhered to each other with plasma; (b) aligning thetreated surfaces to each other; and (c) pressing the treated surfacestogether until a watertight seal is formed around the cavity.
 11. Themethod of claim 1, wherein the adhering is done in a bonding chamber atpressure of no more than 0.001 Torr.
 12. The method of claim 1, furthercomprising, after the adhering, trapping charge in the firstpolymer-based layer by: (a) applying a voltage across the top electrodeand the substrate assembly such that a portion of the firstpolymer-based layer contacting the top electrode is pulled into contactwith the substrate assembly; (b) maintaining the portion of the firstpolymer-based layer contacting the top electrode and the substrateassembly in contact for a period of time; and then (c) ceasing applyingthe voltage.
 13. The method of claim 1, wherein the sacrificial layercomprises a polymer.
 14. The method of claim 1, wherein depositing thesecond polymer-based layer on the sacrificial layer comprises completelycovering the sacrificial layer with the second polymer-based layer. 15.The method of claim 1 wherein the substrate assembly comprises anon-conductive substrate with a conductive bottom electrode on thesubstrate.
 16. The method of claim 1, wherein the substrate assemblycomprises an optically-transparent conductive bottom electrode on anoptically-transparent substrate.
 17. The method of claim 1, wherein thesacrificial layer is non-reactive when exposed to the secondpolymer-based layer and to a photoresist developer used during thepatterning of the second polymer-based layer, and wherein the secondpolymer-based layer is non-reactive when exposed to an etchant used toetch away the sacrificial layer.
 18. The method of claim 1, wherein thefirst, second, and third polymer-based layers comprise SU8 photoresistand the sacrificial layer comprises an OmniCoat™ composition.
 19. Themethod of claim 1, wherein the cavity has a height selected such that anoperating voltage of the transducer is no more than 50 Volts.
 20. Themethod of claim 1, wherein depositing the sacrificial layer comprisesevaporating a composition that comprises a solvent, and then depositingthe composition as the sacrificial layer, wherein at least 70% and nomore than 90% of the solvent is evaporated.